Origin and Evolution of Ore-Forming Fluids in Luerma Porphyry Copper (Gold) Deposit from Western Gangdise

The current studies of the deposits in the Gangdese metallogenic belt is predominate in the eastern section, but the new discovered Luerma porphyry copper (gold) Deposit belongs to the western segment of Gangdise polymetallic metallogenic belt. The Luerma copper (gold) deposit developed typical porphyry deposits' hydrothermal alteration zones, which are divided as potassium-silicification zone, sericitization zone, clayization zone, and propylitization-propylitization zone from proximal to distal, respectively. Hornfels and malachite are also generally distributed in the mining area. Moreover, three main types of hydrothermal veins have been identified based on its mineral assemblages, cutting relationship and alteration features, which consist of the potassium-silicification vein (A vein), the medium-term quartz-polymetallic sulfides vein (B vein), and the epidote-carbonatation (D vein), respectively. Petrographic, microthermometric, laser Raman microprobe and H-O-C results of fluid inclusions in different hydrothermal veins. Fluid inclusions of A vein's homogenization temperatures, and salinities vary from 390~460 ℃, 4.5%~21.6% NaCleqv, 43.6%~59.6% NaCleqv, respectively; fluid inclusions of B vein's homogenization temperatures, and salinities, vary from 310~380 ℃, 3.6%~19.8% NaCleqv, 6.0%~16.0% NaCleqv, respectively; fluid inclusions of D vein's homogenization temperatures, and salinities, vary from 200~320 ℃, 0.4%~14.7% NaCleqv, 0.70~1.00 g/cm3, respectively. And the carbon, hydrogen, oxygen isotope test results reveals that the δDH2O,V-SMOW values of fluid inclusions in quartz veins range from -128‰ to -100‰, and δ18OH2O,V-SMOW values of fluid inclusions in quartz veins range from -9.09 ‰ to -1.45‰, the δ13CCal,V-PDB values of calcite veins range from -20.8‰ to -19.9‰, and δ18OCal,V-SMOW values of calcite veins range 9.4‰ to 10.5‰, indicating a feature of magmatic hydrothermal, but may mixtured geothermal water in late stage. In brief, the ore- forming fluid of the Luerma copper (gold) deposit is a Ca+-Na+- Cl-H2O fluid system, with high contents of CO2, N2, and CH4, high homogenization temperature, high salinity and, low-moderate density, rich in metallic elements as Cu, Fe, and Mo et al., which characteristics similar to typical porphyry copper deposits. These studies suggest that, the luerma copper ore ore-forming fluid moved from the deep closed system to the shallow open system and broke through the critical state of decompressing boiling rapidly, which occurred phase separation resulting in the precipitation of metal sulfide, forming A vein and B vein type mineralization. Afterwards, as the heavy precipitation of minerals in ore bearing hydrothermal fluid, and the mixing of atmospheric precipitation, et al., the temperature and salinity of the fluid decreased rapidly, resulting in D vein mineralization. © 2019, Editorial Department of Earth Science. All right reserved.